1. Field of the Invention
This invention relates to a method of improving the adhesion of a polymeric material to a metal surface and more particularly to the adhesion of epoxy resins to metallic substrates such as are used in the manufacture of electronic printed circuit boards.
2. The Prior Art
In many applications, it is essential to provide strong adherence between a metallic surface and a polymeric surface. Examples of such uses are widely diversified. For example, the strong adherence of metal to polymer is critical in the manufacture of multilayer printed circuit boards in the electronics, telecommunications, computer and other industries.
In the manufacture of electronic devices, such as computers, printed circuit boards are widely used to support discrete electronic components and to provide the electrical circuitry between the components. The printed circuit boards are composite structures having a central core, typically a dielectric material such as glass fiber and a thermosetting resin such as an epoxy, phenolic polyester or other thermosetting resin, referred to in the art as a "prepreg". The prepreg has applied on either side thereof, metallized circuitry, usually formed of a metal film layer such as copper. The metal film layer is etched or otherwise processed to provide circuits of predetermined geometrical configuration. The printed circuit board may be two-sided composite board having circuitry on the top and bottom sides of the dielectric layer, or it may be laminated multilayer board which also contains internal signal lines and power planes separated by the dielectric polymeric material. The laminated multilayer printed circuit boards are prepared from individual composite circuit boards which are laminated to form the multilayer structure having the etched metal circuitry sandwiched between the prepreg layers. The laminated multilayer boards are provided with via holes and interstitial via holes to interconnect the various circuit lines and power planes.
Various methods have been employed for providing a metal film layer on one side of a prepreg substrates which is to be laminated to form a multilayer printed circuit board. Such methods include deposition by vacuum metallization and electroless plating. While electroless plating has become a desirable and cost efficient method of depositing metal film layers such as copper onto polymeric prepreg substrates, the adhesion of such layer after deposition to other polymeric substrates is often less than adequate, especially with respect to polymers such as epoxy resins. Copper, when electroless plated, is in its pure form and like other pure metals, generally exhibits poor adhesion characteristics for bonding to polymeric substrates. For example, it has been determined that in order for a multilayer printed circuit board having copper signal lines sandwiched between dielectric layers formed from epoxy resins to meet commercial specifications, it is required that the interlaminate bond strength between the copper film layer and the opposed dielectric layer be at least about 4.2 pounds pull per inch (lbs/in.). If an attempt to laminate an untreated copper coated epoxy substrate to an epoxy based substrate were made, the interlaminate bond strength would generally be found to be in the range of about 1-2 lbs/in.
To improve the interlaminate bond strength of electroless plated copper films to epoxy resin based dielectrics, it has been the practice in the art to treat the copper film surface prior to lamination with a solution of sodium chlorite and sodium hydroxide at temperatures near boiling to convert the copper film surface to a more adherent copper oxide. Such a solution generally contains 3 to 8% by weight chlorite. This chlorite treatment step generally raises the interlaminate bond strength of the copper film layer to about 5.0-10.0 lbs/in.
A drawback to the chlorite adhesion promoting treatment, which has been used since the early days of printed circuit technology is that the oxide coatings formed are often non-uniform, resulting in areas of poor copper to polymer adhesion after lamination. Particular care must be paid to the thickness of the oxide coating, i.e. it must be very uniform or else poor adhesion will result.
Another drawback to the chlorite treatment step is that the copper metal surface must be cleaned with surfactants and highly corrosive mineral acids prior to immersion in the chlorite solution. The present process does not require such a cleaning pre-step.
A further drawback to the chlorite treatment is that the concentrated solutions used to effect a chemical conversion of copper to copper oxide to promote adhesion also tend to undercut the deposited copper film layer, i.e., laterally undermine the conductor paths on the circuit patterns etched in the prepreg surface.
Another main disadvantage to the use of chlorite solutions for the enhancement of polymer adhesion during lamination to metallized surfaces is that the spent chlorite solutions are hazardous chemicals and create disposal problems which arise from the fact that generally the spent solution cannot be simply discarded without appropriate treatment to avoid pollution, thereby increasing the expense of using the treatment.